1. Field of the Invention
The present invention relates to a magnetic coil and electrical equipment such as a magnetic contactor with which such magnetic coil is used, and more specifically to the structure of a surge absorber installed on such magnetic coil and the mounting structure for such surge absorber.
2. Description of the Background Art
FIG. 20 is a front sectional view of electrical equipment (a magnetic contactor in this case) where a magnetic coil concerned with a first conventional design is housed. In FIG. 20, the numeral 1 indicates a mounting base, 2 represents a case, and 3 denotes a magnetic coil. 4 designates a fixed core which is disposed opposite to a movable core 5 with a predetermined gap provided therebetween. 6 indicates a crossbar made of an isolating material and connected to the movable core 5, of which top window 6a slidably holds a movable contactor 8. The crossbar 6 is slidable guided in FIG. 20 by the case 2 so as to be movable vertically (not shown). 7 indicates a contact spring which uses a compression coil spring and is disposed to provide contact pressure to the movable contactor 8. 8a and 8b denote movable contacts mounted at both ends of the movable contactor 8 and disposed opposite to fixed contacts 9a, 10a, respectively, with a predetermined gap provided therebetween. 9 and 10 indicate terminal plates having the fixed contacts 9a, 10a, respectively, joined on one end and terminal screws 11 threaded on the other end. 12 designates a cover for preventing an arc produced between the contacts from coming out. 13 indicates a trip spring disposed to press the coupled crossbar 6 and movable core 5 upward in FIG. 20.
As shown in a perspective view of FIG. 21, the magnetic coil 3 comprises a resin coil spool 14, a winding 15 wound around said coil spool 14, and a start-of-winding coil terminal 16 and an end-of-winding coil terminal 17 installed on the coil spool 14. The start-of-winding and end-of-winding terminations of the winding 15 are electrically connected to a projection 16b of the start-of-winding coil terminal 16 and a projection 17b of the end-of-winding coil terminal 17, respectively. The other end of the start-of-winding coil terminal 16 and the other end of the end-of-winding coil terminal 17 are exposed to the outside of the magnetic contactor (equipment) and are provided with internally threaded holes 16c and 17c, respectively, into which terminal screws 11 have been threaded. 30 indicates a covering tape which protects and isolates the winding 15.
In FIG. 20, 18 denotes a surge absorber which, as shown in a perspective view of FIG. 22, consists of a surge absorber body 20 housed in a case 19 and absorber terminals 21, 22 connected electrically to both ends of the surge absorber-body 20 and protruding from the case 19. 23 indicates casting resin which secures the surge absorber body 20 and the absorber terminals 21, 22 to the case 19 and electrically isolates the same from each other. As the surge absorber body 20, a voltage-dependent resistor device, e.g., a varistor, or a capacitor-resistor series circuit, etc., is used. An engagement projection 19a provided on the case 19 is engaged with an engagement recess la in the mounting base 1 to mechanically fix the surge absorber 18 arranged as described above to the magnetic contactor. Also, the absorber terminals 21, 22 are fastened to the internally threaded hole 16c in the start-of-winding coil terminal 16 and the internally threaded hole 17c in the end-of-winding coil terminal 17, respectively, by means of the terminal screws 11 to electrically connect the surge absorber 18 in parallel with the magnetic coil winding 15.
Operation will now be described. When a voltage is applied to the magnetic coil 3, resultant magnetic flux produces absorbing force between the fixed core 4 and the movable core 5. This absorbing force causes the coupled movable core 5 and crossbar 6 to move downward in FIG. 20 against the pressing force of the trip spring 13. This movement brings the movable contacts 8a, 8b into contact with the fixed contacts 9a, 10a. Since the core gap in the open state of FIG. 20 is designed to be larger than the contact gap, the crossbar 6 further moves downward below the contact making position to provide contact wipe, and the contact spring 7 is compressed and its force is provided to the movable contactor 8, acting as contact pressure. The contact closing operation is thus completed. Now, when the voltage is removed from the magnetic coil 3, the absorbing force between the movable core 5 and the fixed core 4 disappears and the coupled movable core 5 and crossbar 6 are moved upward by the pressing force of the trip spring 13, causing the contacts to open. At this time, an arc produced between the contacts is extended, cooled and extinguished, whereby the contact opening operation is completed.
When the voltage is removed from the magnetic coil 3 in the above operation of the magnetic contactor, a surge having an extremely high voltage value occurs. However, since this surge is absorbed as described above by the surge absorber 20 connected in parallel with the magnetic coil winding 15, it is possible to prevent malfunction induced by a surge voltage in the electronic circuit (not shown) of the other equipment (not shown) connected in parallel with the magnetic coil.
In a second conventional design, as disclosed in Japanese Patent Disclosure Laid-open No. 40823 of 1990, a surge absorber having a surge absorbing device molded by resin is provided outside a control equipment vessel and the absorber terminals of the surge absorber are electrically connected with coil terminals by elastically pressing the whole surge absorber toward the coil terminals without screw tightening.
In a third conventional design, as disclosed in Japanese Utility Model Disclosure Laid-open No. 20650 of 1989, a clip, which wraps up and grips a surge absorbing device that is electrically connected via coil terminations and lead wires, is engaged with a rib provided on the top surface of a coil spool, whereby the surge absorbing device is held on the top surface of the coil spool.
In the first conventional design, the engagement projection provided on the surge absorber case is engaged with the engagement recess of the mounting base to install the surge absorber on the outside of the magnetic contactor and the absorber terminals are fastened to the coil terminals with the screws to electrically connect the absorber terminals and the coil terminals. In such design, the terminal screws must be loosened or removed and the surge absorber must also be removed and reinstalled when the magnetic coil voltage rating of the magnetic contactor is to be changed, i.e., when the magnetic coil is to be replaced by one having a different voltage rating, resulting in troublesome work. In addition, when a voltage-dependent resistor device, such as a varistor, is used as the surge absorber, the device must have device constants (resistance inflection point voltage value, etc.) corresponding to the magnetic coil voltage rating to protect the device from damage, requiring extreme caution in device selection.
In the second conventional design, the absorber terminals of the surge absorber provided with the resin-molded surge absorbing device at the outside of the control equipment vessel are designed to be electrically connected with the coil terminals by elastically pressing the whole surge absorber toward the coil terminals. Hence, when the magnetic coil is to be replaced by one having a different voltage rating, it is not necessary to loosen or remove the terminal screws. As in the first conventional design, however, when a voltage-dependent resistor device, such as a varistor, is employed as the surge absorber, the device must have device constants (resistance inflection point voltage value, etc.) corresponding to the magnetic coil voltage rating to protect the device from damage, requiring extreme caution when the device is selected.
Also, in the second conventional design, the absorber terminals of the surge absorber mounted on the control equipment vessel are designed to be electrically connected with the coil terminals by elastically pressing the absorber terminals toward the coil terminals. Hence, the absorber terminals electrically connected with the coil terminals may be separated therefrom by vibration produced when the movable core 5 comes into contact with, or moves away from, the fixed core 4, offering low reliability.
In the third conventional design, wherein the surge absorber is designed to be integral with the magnetic coil, the disadvantages of the first and second conventional design can be resolved but it is desired to automate the work of assembling the surge absorber into the magnetic coil if the surge absorber is designed to be integral with the magnetic coil. In the third conventional design, however, the surge absorbing device is held on the top surface of the coil spool by engaging the clip for wrapping and gripping the surge absorbing device electrically connected by the coil terminations and lead wires with the rib provided on the top surface of the coil spool. Therefore, automatic assembling of the surge absorber into the magnetic coil could not be done or was very difficult to be done.
Also, in the third conventional design as described above, if the surge absorbing device is damaged, the whole magnetic coil must be changed, or if only the surge absorbing device is changed, the surge absorbing device must first be desoldered and deprived of the lead wires. Then, after the surge absorbing device is changed, the lead wires must be reconnected, resulting in extremely low economy or changing workability.
Further, the third conventional design does not allow the rating, etc., of the built-in surge absorber to be checked from outside of the equipment.